Making a APO with a flat field

[Jim Fusco]

Andrey,

When creating a apo, what or how can the size of image field is controlled. Does a quad, or triplet design have a advantage over a doublet or is it the glass type more so controls this factor in the design.

[astreya]

Hi Jim,

of course, when creating an apo, designer controls image quality on an axis and with a field. Experienced designer knows what a result he can obtain with this or that optical design: doublet, triplet or some others. It's difficult to design, say, doublet with big field because there are not enough parameters for correction. It is easier to do in triplets. Generally speaking, the more parameters (glasses) in a design, the more possibilities to correct aberrations like coma, astigmatism, lateral colour, etc.
Glass types also should be under control for better colour correction.
There are some optical designes of lenses for imaging if we're speaking about refractors: Petzval is the most known, also - designs with flatteners and the others. I have my own original designes of such lenses.

[Jim Fusco]

Thanks Andrey,

So the flattener, flattens the image. But does a flattener over or under corrects visually or can create aberrations simular to a camera lens?

Jim

[astreya]

Hi Jim,

speaking about flatteners for refractor lens it should be mention that it's impossible to make universal one which can work well with different types of lenses. So, when users combine flattener from one company with a lens from another, the result could be bad. Except spherical aberration, it can bring in chromatism, lateral colour, etc. But if flattener was designed especially for some type of APO lens, the results can be very good. It's possible to obtain image quality close to diffraction limit for rather wide field. For example, see diagrams for the sistem of 90mm ED-doublet plus 2-elements flattener, which I designed recently.

[Jim Fusco]

Very interesting, so what actually controls the field. Is it the curvature of the front or rear element of the of the objective. I was wondering if there were other flattener designs that you created .

Thanks,

Jim

[astreya]

If we're speaking about field curvature like a specific type of monochromatic aberrations, we should say that it is possible to make this curvature equal to zero. For this, designer should execute Petzval condition. This formula includes refraction indexes and radii of all surfaces which are included in the project. But really this aberration doesn't exist separately from others such as astigmatism, coma, etc. So, designer should control all aberrations using all variable parameters and executing other conditions. Sometimes this conditions clash among themselves. So final design is a compromise between different formulas. But the more variable parameters has designer, the better balance among residual aberrations is in final design. In the example I've shown you real curvature of image surface is more than 1 m and practically there is diffraction limited image inside the circle of 35mm diameter. So this design can be used as visual and for imaging. It should be mention that I used only flattener parameters to control field curvature and other aberrations, the lens was designed earlier by other designer.
Yes, I made other flatteners for different types of lenses and also made original designes of wide field lenses for imaging.

[Jim Fusco]

Thanks Andrey,
So it seems a Field Fattener could not enhance a image beyond the optical design of the objective. Is there a difference if a ED glass is used or if the flattener is triplet in design.

Thanks,
Jim

[ghartke]

Hi, Jim,

For visual use, I would expect that you would want well-corrected optics and avoid the field flattener. After all, the addition of extra surfaces will only serve to reduce contrast and throughput. The losses incurred might be significant for visual use but be less of an issue for imaging use. In the latter case, the color correction remains important but the flatness of the field now becomes important if you're using a focal plane of any reasonable size.

Andrey, correct me if I'm wrong but doesn't the radius of curvature of the image plane depend solely on the focal length in a simple refractive system?

Regards,

Greg

[astreya]

Hi Jim, Greg,

really, in a singlet the radius of curvature of the image surface depends solely on the focal length. But the image will be bad both on an axis, and off it.
Any visual sistem produces high quality image on axis and near it only, so you can't use this sistem for imaging with any reasonable size of detector. For this you should use a flattener. It is difficult to expect that spot quality on the axis will be better when using flattener, of course not, but off axis image will be significantly better. And spot diameter is usually more than it is in visual system on axis. It's a price for a big field.
It is possible also to make visual/photo system where spot and field sizes are balanced but in this case, all (both objective and flattener) parameters should be optimized in one design. It can be Petzval or some other design.